This is one of the most fascinating ways to volunteer I've come across yet. UN Volunteering online offers ways for you to "share you skills, knowledge and ideas - from a computer anywhere in the world."
Simply choose a task that matches your skills (research, translation, project development and management, writing and editing, design, IT development, training and coaching, coordination and facilitation, consulting).
Focus on a specific development topic (education, income generation and employment, youth, governance and human rights, health, environment, culture, gender, among others).
Then select a region in the world.
My contribution: I registered online, it took less than a minute. They'll send you a confirmation e-mail and then you can set up a profile that matches your interests with their opportunities. I'm very interested to see where this one leads me.